Birth time/order-dependent neuron type specification

Deterministic and probabilistic fate decisions co-exist in a single retinal lineage

Correct nervous system development depends on the timely differentiation of progenitor cells into neurons. While the output of progenitor differentiation is well investigated at the population and clonal level, how stereotypic or variable fate decisions are during development is still more elusive. To fill this gap, we here follow the fate outcome of single neurogenic progenitors in the zebrafish retina over time using live imaging. We find that neurogenic progenitor divisions produce two daughter cells, one of deterministic and one of probabilistic fate. Interference with the deterministic branch of the lineage affects lineage progression. In contrast, interference with fate probabilities of the probabilistic branch results in a broader range of fate possibilities than in wild-type and involves the production of any neuronal cell type even at non-canonical developmental stages. Combining the interference data with stochastic modelling of fate probabilities revealed that a simple gene regulatory network is able to predict the observed fate decision probabilities during wild-type development. These findings unveil unexpected lineage flexibility that could ensure robust development of the retina and other tissues.

Timely Inhibitory Circuit Formation Controlled by Abl1 Regulates Innate Olfactory Behaviors in Mouse

More than one-half of the interneurons in a mouse olfactory bulb (OB) develop during the first week after birth and predominantly connect to excitatory tufted cells near the superficial granule cell layer (sGCL), unlike late-born interneurons. However, the molecular mechanisms underlying the temporal specification are yet to be identified. In this study, we determined the role of Abelson tyrosine-protein kinase 1 (Abl1) in the temporal development of early-born OB interneurons. Lentiviral knockdown of Abl1 disrupts the sGCL circuit of early-born interneurons through defects in function and circuit integration, resulting in olfactory hyper-sensitivity. We show that doublecortin (Dcx) is phosphorylated by Abl1, which contributes to the stabilization of Dcx, thereby regulating microtubule dynamics. Finally, Dcx overexpression rescues Abl1 knockdown-induced anatomic or functional defects. In summary, specific signaling by Abl1-Dcx in early-born interneurons facilitates the temporal development of the sGCL circuit to regulate innate olfactory functions, such as detection and sensitivity.

Gene regulatory networks in Drosophila early embryonic development as a model for the study of the temporal identity of neuroblasts

Genes belonging to the "gap" and "gap-like" family constitute the best-studied gene regulatory networks (GRNs) in Drosophila embryogenesis. Gap genes are a core of two subnetworks controlling embryonic segmentation: (hunchback, hb; Krüppel, Kr; giant, gt; and knirps, kni) and (hb; Kr; pou-domain, pdm; and, probably, castor, cas). Of particular interest is that (hb, Kr, pdm, cas) also specifies the temporal identity of stem cells, neuroblasts, in Drosophila neurogenesis. This GRN controls the sequential differentiation of neuroblasts during the asymmetric cell division. In the last decades, modeling of the patterning of gene ensemble (hb, Kr, gt, kni) in segmentation was in the center of attention. We show that our previously published and extensively studied model at a certain level of external factors is able to reproduce temporal patterns of (hb, Kr, pdm, cas) in neurogenesis with minor evolutionary explicable modifications. This result testifies in favor of a hypothesis that the similarity of two gene ensembles active in segmentation and neurogenesis is a result of co-option of the network architecture in evolution from the common ancestral form. By means of the model dynamical analysis, it is shown that the establishment of the robust patterns in both systems could be explained in terms of the action of attractors in the gap gene dynamical system. We formulate the common principles underlying the robustness of both GRNs in segmentation and neurogenesis due to the similar functional organization of the gene ensembles as having the same evolutionary origin.

Distinct Developmental Features of Olfactory Bulb Interneurons

The olfactory bulb (OB) has an extremely higher proportionof interneurons innervating excitatory neurons than otherbrain regions, which is evolutionally conserved across species.Despite the abundance of OB interneurons, little is knownabout the diversification and physiological functions ofOB interneurons compared to cortical interneurons. In thisreview, an overview of the general developmental processof interneurons from the angles of the spatial and temporalspecifications was presented. Then, the distinct featuresshown exclusively in OB interneurons development andmolecular machinery recently identified were discussed.Finally, we proposed an evolutionary meaning for thediversity of OB interneurons.

The role of cell lineage in the development of neuronal circuitry and function

Complex nervous systems have a modular architecture, whereby reiterative groups of neurons ("modules") that share certain structural and functional properties are integrated into large neural circuits. Neurons develop from proliferating progenitor cells that, based on their location and time of appearance, are defined by certain genetic programs. Given that genes expressed by a given progenitor play a fundamental role in determining the properties of its lineage (i.e., the neurons descended from that progenitor), one efficient developmental strategy would be to have lineages give rise to the structural modules of the mature nervous system. It is clear that this strategy plays an important role in neural development of many invertebrate animals, notably insects, where the availability of genetic techniques has made it possible to analyze the precise relationship between neuronal origin and differentiation since several decades. Similar techniques, developed more recently in the vertebrate field, reveal that functional modules of the mammalian cerebral cortex are also likely products of developmentally defined lineages. We will review studies that relate cell lineage to circuitry and function from a comparative developmental perspective, aiming at enhancing our understanding of neural progenitors and their lineages, and translating findings acquired in different model systems into a common conceptual framework.

Analysis of Complete Neuroblast Cell Lineages in the Drosophila Embryonic Brain via DiI Labeling

Proper functioning of the brain relies on an enormous diversity of neural cells generated by neural stem cell-like neuroblasts (NBs). Each of the about 100 NBs in each side of brain generates a nearly invariant and unique cell lineage, consisting of specific neural cell types that develop in defined time periods. In this chapter we describe a method that labels entire NB lineages in the embryonic brain. Clonal DiI labeling allows us to follow the development of an NB lineage starting from the neuroectodermal precursor cell up to the fully developed cell clone in the first larval instar brain. We also show how to ablate individual cells within an NB clone, which reveals information about the temporal succession in which daughter cells are generated. Finally, we describe how to combine clonal DiI labeling with fluorescent antibody staining that permits relating protein expression to individual cells within a labeled NB lineage. These protocols make it feasible to uncover precise lineage relationships between a brain NB and its daughter cells, and to assign gene expression to individual clonal cells. Such lineage-based information is a critical key for understanding the cellular and molecular mechanisms that underlie specification of cell fates in spatial and temporal dimension in the embryonic brain.

New Insights Into Cholinergic Neuron Diversity

Cholinergic neurons comprise a small population of cells in the striatum but have fundamental roles in fine tuning brain function, and in the etiology of neurological and psychiatric disorders such as Parkinson’s disease (PD) or schizophrenia. The process of developmental cell specification underlying neuronal identity and function is an area of great current interest. There has been significant progress in identifying the developmental origins, commonalities in molecular markers, and physiological properties of the cholinergic neurons. Currently, we are aware of a number of key factors that promote cholinergic fate during development. However, the extent of cholinergic cell diversity is still largely underestimated. New insights into the biological basis of their specification indicate that cholinergic neurons may be far more diverse than previously thought. This review article, highlights the physiological features and the synaptic properties that segregate cholinergic cell subtypes. It provides an accurate picture of cholinergic cell diversity underlying their organization and function in neuronal networks. This review article, also discusses current challenges in deciphering the logic of the cholinergic cell heterogeneity that plays a fundamental role in the control of neural processes in health and disease.

The Hunchback temporal transcription factor determines motor neuron axon and dendrite targeting in Drosophila

The generation of neuronal diversity is essential for circuit formation and behavior. Morphological differences in sequentially born neurons could be due to intrinsic molecular identity specified by temporal transcription factors (henceforth called intrinsic temporal identity) or due to changing extrinsic cues. Here, we have used the Drosophila NB7-1 lineage to address this issue. NB7-1 generates the U1-U5 motor neurons sequentially; each has a distinct intrinsic temporal identity due to inheritance of different temporal transcription factors at its time of birth. We show that the U1-U5 neurons project axons sequentially, followed by sequential dendrite extension. We misexpressed the earliest temporal transcription factor, Hunchback, to create 'ectopic' U1 neurons with an early intrinsic temporal identity but later birth-order. These ectopic U1 neurons have axon muscle targeting and dendrite neuropil targeting that are consistent with U1 intrinsic temporal identity, rather than with their time of birth or differentiation. We conclude that intrinsic temporal identity plays a major role in establishing both motor axon muscle targeting and dendritic arbor targeting, which are required for proper motor circuit development.

A Notch-mediated, temporal asymmetry in BMP pathway activation promotes photoreceptor subtype diversification

Neural progenitors produce neurons whose identities can vary as a function of the time that specification occurs. Here, we describe the heterochronic specification of two photoreceptor (PhR) subtypes in the zebrafish pineal gland. We find that accelerating PhR specification by impairing Notch signaling favors the early fate at the expense of the later fate. Using in vivo lineage tracing, we show that most pineal PhRs are born from a fate-restricted progenitor. Furthermore, sister cells derived from the division of PhR-restricted progenitors activate the bone morphogenetic protein (BMP) signaling pathway at different times after division, and this heterochrony requires Notch activity. Finally, we demonstrate that PhR identity is established as a function of when the BMP pathway is activated. We propose a novel model in which division of a progenitor with restricted potential generates sister cells with distinct identities via a temporal asymmetry in the activation of a signaling pathway.

A major goal in the field of developmental neurobiology is to identify the mechanisms that underly the diversification of the subtypes of neurons that are needed for the function of the nervous system. When investigating these mechanisms, time is an often-overlooked variable. Here, we show that in the zebrafish pineal gland—a neuroendocrine organ containing mostly photoreceptors (PhRs) and projection neurons—different classes of PhRs appear in a temporal sequence. In this simple system, the decision to adopt a PhR fate is driven by the activation of the bone morphogenetic protein (BMP) signaling pathway. Following the final cell division of a PhR progenitor, the sister cells normally activate the BMP pathway at different times. When Notch signaling activity is abrogated, activation of the BMP pathway occurs earlier and synchronously, which in turn favors the development of early PhR fates at the expense of later fates. We propose a model in which preventing sister cells from activating a signaling pathway in a synchronous fashion after their final division allows diversification of cell fates.

Transitional Progenitors during Vertebrate Retinogenesis

The retina is a delicate neural tissue responsible for light signal capturing, modulating, and passing to mid-brain. The brain then translated the signals into three-dimensional vision. The mature retina is composed of more than 50 subtypes of cells, all of which are developed from a pool of early multipotent retinal progenitors, which pass through sequential statuses of oligopotent, bipotent, and unipotent progenitors, and finally become terminally differentiated retinal cells. A transitional progenitor model is proposed here to describe how intrinsic developmental programs, along with environmental cues, control the step-by-step differentiation during retinogenesis. The model could elegantly explain many current findings as well as predict roles of intrinsic factors during retinal development.

Cell-cycle-independent transitions in temporal identity of mammalian neural progenitor cells

During cerebral development, many types of neurons are sequentially generated by self-renewing progenitor cells called apical progenitors (APs). Temporal changes in AP identity are thought to be responsible for neuronal diversity; however, the mechanisms underlying such changes remain largely unknown. Here we perform single-cell transcriptome analysis of individual progenitors at different developmental stages, and identify a subset of genes whose expression changes over time but is independent of differentiation status. Surprisingly, the pattern of changes in the expression of such temporal-axis genes in APs is unaffected by cell-cycle arrest. Consistent with this, transient cell-cycle arrest of APs in vivo does not prevent descendant neurons from acquiring their correct laminar fates. Analysis of cultured APs reveals that transitions in AP gene expression are driven by both cell-intrinsic and -extrinsic mechanisms. These results suggest that the timing mechanisms controlling AP temporal identity function independently of cell-cycle progression and Notch activation mode.

Conserved patterns of axogenesis in the panarthropod brain

Neuropils in the cerebral midline of Panarthropoda exhibit a wide spectrum of neuroarchitectures--from rudimentary to highly elaborated--and which at first sight defy a unifying neuroarchitectural principle. Developmental approaches have shown that in model arthropods such as insects, conserved cellular and molecular mechanisms first establish a simple axon scaffold in the brain. However, to be adapted for adult life, this immature ground plan is transformed by a developmental process--known in the grasshopper as "fascicle switching"--in which subsets of neurons systematically redirect their growth cones at stereotypic locations across the brain midline. A topographic system of choice points along the transverse brain axis where axons decussate features in all panarthropods studied even though different modes of neurogenesis and varying degrees of neuropilar elaboration are involved. This suggests that the molecular mechanisms regulating choice point selection may be conserved. In combination with recent cladistic interpretations of arthropod phylogeny based on nuclear protein-coding sequences the data argue for this topographic decussation as having evolved early and being a conserved feature of the Panarthropoda. Differences in elaboration likely reflect both the extent to which neuropilar reorganization has progressed during development and the lifestyle of the individual organism.

Convergent microRNA actions coordinate neocortical development

Neocortical development is a complex process that, at the cellular level, involves tight control of self-renewal, cell fate commitment, survival, differentiation and delamination/migration. These processes require, at the molecular level, the precise regulation of intrinsic signaling pathways and extrinsic factors with coordinated action in a spatially and temporally specific manner. Transcriptional regulation plays an important role during corticogenesis; however, microRNAs (miRNAs) are emerging as important post-transcriptional regulators of various aspects of central nervous system development. miRNAs are a class of small, single-stranded noncoding RNA molecules that control the expression of the majority of protein coding genes (i.e., targets). How do different miRNAs achieve precise control of gene networks during neocortical development? Here, we critically review all the miRNA–target interactions validated in vivo, with relevance to the generation and migration of pyramidal-projection glutamatergic neurons, and for the initial formation of cortical layers in the embryonic development of rodent neocortex. In particular, we focus on convergent miRNA actions, which are still a poorly understood layer of complexity in miRNA signaling, but potentially one of the keys to disclosing how miRNAs achieve the precise coordination of complex biological processes such as neocortical development.

Diverse neuronal lineages make stereotyped contributions to the Drosophila locomotor control center, the central complex

The Drosophila central brain develops from a fixed number of neuroblasts. Each neuroblast makes a clone of neurons that exhibit common trajectories. Here we identified 15 distinct clones that carry larval-born neurons innervating the Drosophila central complex (CX), which consists of four midline structures including the protocerebral bridge (PB), fan-shaped body (FB), ellipsoid body (EB), and noduli (NO). Clonal analysis revealed that the small-field CX neurons, which establish intricate projections across different CX substructures, exist in four isomorphic groups that respectively derive from four complex posterior asense-negative lineages. In terms of the region-characteristic large-field CX neurons, we found that two lineages make PB neurons, 10 lineages produce FB neurons, three lineages generate EB neurons, and two lineages yield NO neurons. The diverse FB developmental origins reflect the discrete input pathways for different FB subcompartments. Clonal analysis enlightens both development and anatomy of the insect locomotor control center.

Analysis of complete neuroblast cell lineages in the Drosophila embryonic brain via DiI labeling

Proper functioning of the brain relies on an enormous diversity of neural cells generated by neural stem cell-like neuroblasts (NBs). Each of the about 100 NBs in each side of brain generates a nearly invariant and unique cell lineage, consisting of specific neural cell types that develop in defined time periods. In this chapter we describe a method that labels entire NB lineages in the embryonic brain. Clonal DiI labeling allows us to follow the development of a NB lineage starting from the neuroectodermal precursor cell up to the fully developed cell clone in the first larval instar brain. We also show how to ablate individual cells within a NB clone, which reveals information about the temporal succession in which daughter cells are generated. Finally, we describe how to combine clonal DiI labeling with fluorescent antibody staining that permits relating protein expression to individual cells within a labeled NB lineage. These protocols make it feasible to uncover precise lineage relationships between a brain NB and its daughter cells, and to assign gene expression to individual clonal cells. Such lineage-based information is a critical key for understanding the cellular and molecular mechanisms that underlie specification of cell fates in spatial and temporal dimension in the embryonic brain.

Extremes of lineage plasticity in the Drosophila brain

An often-overlooked aspect of neural plasticity is the plasticity of neuronal composition, in which the numbers of neurons of particular classes are altered in response to environment and experience. The Drosophila brain features several well-characterized lineages in which a single neuroblast gives rise to multiple neuronal classes in a stereotyped sequence during development. We find that in the intrinsic mushroom body neuron lineage, the numbers for each class are highly plastic, depending on the timing of temporal fate transitions and the rate of neuroblast proliferation. For example, mushroom body neuroblast cycling can continue under starvation conditions, uncoupled from temporal fate transitions that depend on extrinsic cues reflecting organismal growth and development. In contrast, the proliferation rates of antennal lobe lineages are closely associated with organismal development, and their temporal fate changes appear to be cell cycle-dependent, such that the same numbers and types of uniglomerular projection neurons innervate the antennal lobe following various perturbations. We propose that this surprising difference in plasticity for these brain lineages is adaptive, given their respective roles as parallel processors versus discrete carriers of olfactory information.

Unravelling stem cell dynamics by lineage tracing

During embryonic and postnatal development, the different cells types that form adult tissues must be generated and specified in a precise temporal manner. During adult life, most tissues undergo constant renewal to maintain homeostasis. Lineage-tracing and genetic labelling technologies are beginning to shed light on the mechanisms and dynamics of stem and progenitor cell fate determination during development, tissue maintenance and repair, as well as their dysregulation in tumour formation. Statistical approaches, based on proliferation assays and clonal fate analyses, provide quantitative insights into cell kinetics and fate behaviour. These are powerful techniques to address new questions and paradigms in transgenic mouse models and other model systems.

Temporal patterning of neural progenitors in Drosophila

Drosophila has recently become a powerful model system to understand the mechanisms of temporal patterning of neural progenitors called neuroblasts (NBs). Two different temporal sequences of transcription factors (TFs) have been found to be sequentially expressed in NBs of two different systems: the Hunchback, Krüppel, Pdm1/Pdm2, Castor, and Grainyhead sequence in the Drosophila ventral nerve cord; and the Homothorax, Klumpfuss, Eyeless, Sloppy-paired, Dichaete, and Tailless sequence that patterns medulla NBs. In addition, the intermediate neural progenitors of type II NB lineages are patterned by a different sequence: Dichaete, Grainyhead, and Eyeless. These three examples suggest that temporal patterning of neural precursors by sequences of TFs is a common theme to generate neural diversity. Cross-regulations, including negative feedback regulation and positive feedforward regulation among the temporal factors, can facilitate the progression of the sequence. However, there are many remaining questions to understand the mechanism of temporal transitions. The temporal sequence progression is intimately linked to the progressive restriction of NB competence, and eventually determines the end of neurogenesis. Temporal identity has to be integrated with spatial identity information, as well as with the Notch-dependent binary fate choices, in order to generate specific neuron fates.

Steroid-induced microRNA let-7 acts as a spatio-temporal code for neuronal cell fate in the developing Drosophila brain

Mammalian neuronal stem cells produce multiple neuron types in the course of an individual's development. Similarly, neuronal progenitors in the Drosophila brain generate different types of closely related neurons that are born at specific time points during development. We found that in the post-embryonic Drosophila brain, steroid hormones act as temporal cues that specify the cell fate of mushroom body (MB) neuroblast progeny. Chronological regulation of neurogenesis is subsequently mediated by the microRNA (miRNA) let-7, absence of which causes learning impairment due to morphological MB defects. The miRNA let-7 is required to regulate the timing of α'/β' to α/β neuronal identity transition by targeting the transcription factor Abrupt. At a cellular level, the ecdysone-let-7-Ab signalling pathway controls the expression levels of the cell adhesion molecule Fasciclin II in developing neurons that ultimately influences their differentiation. Our data propose a novel role for miRNAs as transducers between chronologically regulated developmental signalling and physical cell adhesion.

Hierarchical deployment of factors regulating temporal fate in a diverse neuronal lineage of the Drosophila central brain

The anterodorsal projection neuron lineage of Drosophila melanogaster produces 40 neuronal types in a stereotypic order. Here we take advantage of this complete lineage sequence to examine the role of known temporal fating factors, including Chinmo and the Hb/Kr/Pdm/Cas transcriptional cascade, within this diverse central brain lineage. Kr mutation affects the temporal fate of the neuroblast (NB) itself, causing a single fate to be skipped, whereas Chinmo null only elicits fate transformation of NB progeny without altering cell counts. Notably, Chinmo operates in two separate windows to prevent fate transformation (into the subsequent Chinmo-indenpendent fate) within each window. By contrast, Hb/Pdm/Cas play no detectable role, indicating that Kr either acts outside of the cascade identified in the ventral nerve cord or that redundancy exists at the level of fating factors. Therefore, hierarchical fating mechanisms operate within the lineage to generate neuronal diversity in an unprecedented fashion.

Cell biological regulation of division fate in vertebrate neuroepithelial cells

The developing nervous system derives from neuroepithelial progenitor cells that divide to generate all of the mature neuronal types. For the proper complement of cell types to form, the progenitors must produce postmitotic cells, yet also replenish the progenitor pool. Progenitor divisions can be classified into three general types: symmetric proliferative (producing two progenitors), asymmetric neurogenic (producing one progenitor and one postmitotic cell), and symmetric neurogenic (producing two postmitotic cells). The appropriate ratios for these modes of cell division require intrinsic polarity, which is one of the characteristics that define neuroepithelial progenitor cells. The type of division an individual progenitor undergoes can be influenced by cellular features, or behaviors, which are heterogeneous within the population of progenitors. Here we review three key cellular parameters, asymmetric inheritance, cell cycle kinetics, and interkinetic nuclear migration, and the possible mechanisms for how these features influence progenitor fates.

Orthodenticle is necessary for survival of a cluster of clonally related dopaminergic neurons in the Drosophila larval and adult brain

BACKGROUND

The dopaminergic (DA) neurons present in the central brain of the Drosophila larva are spatially arranged in stereotyped groups that define clusters of bilaterally symmetrical neurons. These clusters have been classified according to anatomical criteria (position of the cell bodies within the cortex and/or projection pattern of the axonal tracts). However, information pertaining to the developmental biology, such as lineage relationship of clustered DA neurons and differential cell subtype-specific molecular markers and mechanisms of differentiation and/or survival, is currently not available.

RESULTS

Using MARCM and twin-spot MARCM techniques together with anti-tyrosine hydroxylase immunoreactivity, we have analyzed the larval central brain DA neurons from a developmental point of view and determined their time of birth, their maturation into a DA neurotransmitter phenotype as well as their lineage relationships. In addition, we have found that the homeodomain containing transcription factor Orthodenticle (Otd) is present in a cluster of clonally related DA neurons in both the larval and adult brain. Taking advantage of the otd hypomorphic mutation ocelliless (oc) and the oc2-Gal4 reporter line, we have studied the involvement of orthodenticle (otd) in the survival and/or cell fate specification of these post-mitotic neurons.

CONCLUSIONS

Our findings provide evidence of the presence of seven neuroblast lineages responsible for the generation of the larval central brain DA neurons during embryogenesis. otd is expressed in a defined group of clonally related DA neurons from first instar larvae to adulthood, making it possible to establish an identity relationship between the larval DL2a and the adult PPL2 DA clusters. This poses otd as a lineage-specific and differential marker of a subset of clonally related DA neurons. Finally, we show that otd is required in those DA neurons for their survival.

Reconstruction of rat retinal progenitor cell lineages in vitro reveals a surprising degree of stochasticity in cell fate decisions

In vivo cell lineage-tracing studies in the vertebrate retina have revealed that the sizes and cellular compositions of retinal clones are highly variable. It has been challenging to ascertain whether this variability reflects distinct but reproducible lineages among many different retinal progenitor cells (RPCs) or is the product of stochastic fate decisions operating within a population of more equivalent RPCs. To begin to distinguish these possibilities, we developed a method for long-term videomicroscopy to follow the lineages of rat perinatal RPCs cultured at clonal density. In such cultures, cell-cell interactions between two different clones are eliminated and the extracellular environment is kept constant, allowing us to study the cell-intrinsic potential of a given RPC. Quantitative analysis of the reconstructed lineages showed that the mode of division of RPCs is strikingly consistent with a simple stochastic pattern of behavior in which the decision to multiply or differentiate is set by fixed probabilities. The variability seen in the composition and order of cell type genesis within clones is well described by assuming that each of the four different retinal cell types generated at this stage is chosen stochastically by differentiating neurons, with relative probabilities of each type set by their abundance in the mature retina. Although a few of the many possible combinations of cell types within clones occur at frequencies that are incompatible with a fully stochastic model, our results support the notion that stochasticity has a major role during retinal development and therefore possibly in other parts of the central nervous system.